Mechanical systems such as industrial machineries, vehicles and refrigerating equipments have a variety of sliding parts such as various bearings, pistons, cylinders, gears and drive systems in which frictions are constantly repeated. If poor lubrications occur in these sliding parts, lubrication systems or mechanical systems including the same are no longer able to work as desired. Therefore, various lubricating oils are used as lubricants in the sliding parts depending on the operating environments and conditions.
High stability is required for these lubricating oils from the viewpoint of long-term reliability, but acidic substances are generated due to thermal and oxidative deterioration along with use of the lubricating oils. Since the generated acidic substances exerts a so-called autocatalytic effect, promoting further deterioration, the stability of the lubricants can be greatly increased by removing the acidic substances at an early stage.
In the field of refrigerating equipments, there are movements of transition from current hydrofluorocarbon (HFC) refrigerants having a high global warming potential (GWP) to hydrofluoroolefin (FIFO) refrigerants having a 1/100 or less GWP, from the viewpoint of the prevention of global warming, and a wide range of investigations such as mixed refrigerants containing HFO is underway.
However, since these HFO refrigerants have a low GWP, that is, are easily degradable, hydrofluoric acid (HF) that is a strong acid is therefore generated when degraded in the co-presence of lubricating oils in a refrigeration cycle, and promotes the further deterioration of the lubricating oils and refrigerants, or may cause corrosive wear in some cases to become the cause of system troubles. Thus, it is a major object to increase the stability of a mixed fluid of refrigerants and lubricating oils by effectively removing hydrofluoric acid and acidic substances generated due to the deterioration of lubricating oils.
Since dichlorodifluoromethane (R12) and hydrochlorodifluoromethane (R22) have been largely used as a conventional refrigerant in refrigeration and air-conditioning systems such as refrigerators, car air conditioners and room air conditioners and hydrochloric acid is generated when these refrigerants containing chlorine are mixed with lubricating oils of compressors and deteriorated after used for a long time, it is known to add phenyl glycidyl ether and alkylene oxide compounds which capture hydrochloric acid in the molecules to lubricating oils for compressors (refrigerating machine oils).
Thereafter, the stability of refrigerating machine oils has been attempted to be further improved; for example, Patent Literature 1 shows a refrigerating machine oil containing a glycidyl ether compound having two or more epoxy groups in the molecule; Patent Literature 2 proposes a lubricating oil wherein a phosphorus-based additive and a specific epoxy compound are added in combination therewith; and Patent Literature 3 proposes a refrigerating machine oil to which tricresyl phosphate and an epoxy constituted by glycidyl ether or a carbodiimide are added. Further, Patent Literature 4 shows a refrigerating machine oil containing an alicyclic polycarboxylic acid ester compound as a lubricating base oil, and additionally at least one of an epoxy compound selected from the group consisting of a glycidyl ester type epoxy compound and an alicyclic epoxy compound.
However, these stabilizing agents have a purpose of capturing hydrochloric acid or suppressing the hydrolysis of an ester that is a lubricating base oil, and their effects are insufficient for a novel HFO refrigerant generating hydrofluoric acid that is a strong acid. Moreover, since organic materials such as plastics have come to be largely used in recent devices, for reductions in size and weight and higher efficiency, a stabilizing agent exhibiting a high effect even at a small amount thereof added is desired in order to reduce influences on these organic materials.